1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of fabricating a liquid crystal display device, and more particularly, to a transflective liquid crystal display device having a dual thickness color filter and a method of fabricating the same.
2. Description of Related Art
Presently, liquid crystal display (LCD) devices having light weight, thin profiles, and low power consumption characteristics are commonly used in office automation equipment and video units. The LCD devices typically use a liquid crystal (LC) interposed between upper and lower substrates, and make use of optical anisotropy of the LC. Since molecules of the LC are thin and long, an alignment direction of the LC molecules can be controlled by application of an electric field to the LC molecules. When the alignment direction of the LC molecules is properly adjusted, the LC can be aligned such that light is refracted along the alignment direction of the LC molecules to display images.
In general, LCD devices are divided into transmissive-type LCD devices and reflective-type LCD devices according to whether the display device uses an internal or external light source. The transmissive-type LCD device includes an LCD panel and a backlight device, wherein the incident light produced by the backlight device is attenuated during transmission so that the actual transmittance is only about 7%. In addition, the transmissive-type LCD device requires a relatively high initial brightness, whereby electrical power consumption by the backlight device increases. Accordingly, a relatively heavy battery, which cannot be used for an extended period of time, is needed to supply sufficient power to the backlight device.
In order to overcome these problems, the reflective-type LCD has been developed. Since the reflective-type LCD device uses ambient light instead of the backlight device, wherein a reflective opaque material is used as a pixel electrode, the reflection-type LCD device is light and easy to carry. In addition, since the power consumption of the reflective-type LCD device is reduced, it can be used as a personal digital assistant (PDA). However, the reflective-type LCD device is easily affected by its surroundings. For example, since ambient light in an office differs largely from that of the outdoors, the reflective-type LCD device can not be used where the ambient light is weak or does not exist. In order to overcome the problems described above, a transflective-type LCD device has been developed, wherein a user's may select from the transmissive mode to the reflective mode, or vise versa.
FIG. 1 is a schematic cross sectional view of a transflective-type LCD device according to the related art. In FIG. 1, a transflective-type LCD device includes an upper substrate 10, a lower substrate 30, an interposed liquid crystal layer 20 therebetween, and a backlight device 45 disposed below the lower substrate 30, wherein each of the upper and lower substrates 10 and 30 has a transparent substrate 1. The upper substrate 10 includes a color filter 12 formed on a rear surface of the transparent substrate 1, and an upper transparent electrode 14 formed on the color filter 12, wherein the upper transparent electrode 14 serves as a common electrode. In addition, an upper polarizer 16 is formed on a front surface of the transparent substrate 1, wherein the upper polarizer 16 serves as a filter for selectively transmitting portions of incident light produced by the backlight device 45. Accordingly, the upper polarizer 16 has an optical polarizing axis along one direction such that only the portions of incident light having the same vibrating direction as the direction of the polarizing axis can pass through the upper polarizer 16.
In FIG. 1, the lower substrate 30 includes an insulating layer 33 formed on the front surface of the transparent substrate 1, and a lower transparent electrode 32 formed on the insulating layer 33. In addition, a passivation layer 34 and a reflective electrode 36 are formed in series on the lower transparent electrode 32, and a transmitting hole 31 is formed in the passivation layer 34 and the reflective electrode 36 to expose a portion of the pixel electrode 32. Furthermore, a lower polarizer 40 is formed on the lower surface of the transparent substrate 1 in the lower substrate 30. Thus, when an electric field is applied across the liquid crystal layer 20, molecules of the liquid crystal layer 20 align in accordance with the electric field such that the liquid crystal layer 20 refracts the incident light in order to display an image.
In FIG. 1, an area corresponding to the reflective plate 36 is a reflective portion “r” and an area corresponding to the portion of the pixel electrode 32 exposed by the transmissive hole 31 is a transmissive portion “t”. In addition, a first cell gap “d1” at the transmissive portion “t” is about twice that of a second cell gap “d2” at the reflective portion “r,” thereby reducing a light path difference. A retardation “δ” of the liquid crystal layer 20 is defined as a product of refractive index anisotropy “Δn” with a cell gap “d” (i.e., δ=Δn·d ), wherein a light efficiency of the LCD device is proportional to the retardation “δ.” Accordingly, in order to reduce the difference of light efficiencies between the reflective and transmissive modes, the retardations of the liquid crystal layer 20 at two portions should be nearly equal to each other by making the first cell gap “d1” of the transmissive portion “t” larger than that of the reflective portion “r.”
However, although the light efficiencies of the liquid crystal layer 20 between the reflective and transmissive modes become equal by making the cell gaps different, the light passing through the color filters at different locations is different, wherein the brightness can be different at the front of the display device. The transmittance of the color filter resin whose absorption coefficient is high for a specific wavelength and low for other wavelengths has the following relationship considering only the absorption, i.e., the transmittance is inversely proportional to the absorption coefficient and the distance that light passes:T=exp(−α(λ)d)where T is transmittance of the light, α(λ) is an absorption coefficient depending on the wavelength of the light, and d is a distance that the light passes.
Since the color filter resin is a viscous material, the thickness of the color filter resin is hard to control and can not be fabricated at less than a specific thickness. Therefore, the color filter layers of the reflective and transmissive portions have the same thickness and different absorption coefficients (i.e., different material) for the uniform transmittance. However, if the color filter layers of the reflective and transmissive portions are formed of different materials, the process time and production costs would increase, thereby decreasing yield of the display device.
To solve the above problems, a fabricating method of color filter layers using the same resin has been suggested. During the fabricating method, the color filter layers at the reflective and transmissive portions have the same absorption coefficient, but have different thicknesses so that the transmittance has the same value.
FIG. 2A is a transmittance spectrum measured during a reflective mode of a first red color filter layer having a certain thickness according to the related art, and FIG. 2B is a transmittance spectrum measured during a reflective mode of a second red color filter layer having twice the certain thickness according to the related art. In general, visible light has a wavelength with a range of about 400 to about 700 nanometers, wherein red, green, and blue colors roughly correspond to wavelengths of 650, 550, and 450 nanometers, respectively.
In FIG. 2A, the transmittances at wavelengths corresponding to the red, green, and blue colored light are about 97%, 20% and 58%, respectively. Although the transmittance for the red colored light is high, the transmittances for the other colors are not negligible such that color purity is not obtained.
In FIG. 2B, since the second red color filter layer has twice the thickness and a square transmittance compared with the first red color filter layer of FIG. 2A, the transmittances at wavelengths corresponding to the red, green, and blue colored light are about 94%, 4% and 34%, respectively. Although the transmittance is decreased for all colors, the decreased amount is different for the individual colors, for example, about 5%, 16% and 24% for the red, green, and blue colored lights, respectively. Therefore, the color purity of the second red color filter layer is improved and results can be applied for the green and blue color filters so that the transmittance and color purity of the transflective-type LCD device using the same kind of color filter resin can be uniform for the reflective and transmissive portions. An example of a transflective-type LCD device having a dual thickness color filter (DCF) using the above-detailed principles may be found in Korean Patent Application No. 2000-9979.
FIG. 3 is a cross sectional view of a transflective-type LCD device having a dual thickness color filter layer according to the related art. In FIG. 3, a transparent buffer layer 64 is formed on an inner surface of an upper substrate 15 only at a reflective portion “rr,” and a color filter layer 62 is formed along an entire upper substrate 15. Accordingly, a color filter layer 62 of a transmissive portion “tt” is thicker than that of the reflective portion “rr” so that the color purity of the transmissive portion “tt” can be improved. The transparent buffer layer 64 is formed by depositing and patterning one of an insulating material group comprising acrylic resin, benzocyclobutene (BCB), and silicon nitride (SiNx).
FIG. 4A is a cross sectional view of a dual thickness color filter substrate having a transparent buffer layer of a first thicknesses according to the related art, and FIG. 4B is a cross-sectional view of a dual thickness color filter substrate having a transparent buffer layer of a second thickness according to the related art. In FIG. 4A, a substrate 15 has a transmissive portion “tt” and a reflective portion “rr.” In addition, a black matrix 70 and a transparent buffer layer 64 are formed in the reflective portion “rr,” and a color filter layer 62 is formed along an entire surface of the substrate 15. Since the transparent buffer layer 64 of a first thickness has a low step at a borderline of the transmissive portion “tt” and the reflective portion “rr,” a surface of the color filter layer 62 can be planarized. Moreover, since the color filter layer 62 at the transmissive portion “tt” is thicker than that at the reflective portion “rr”, the color purity can be improved at the transmissive portion “tt”. However, since the thickness of the transparent buffer layer 64 is limited for the planarization of the color filter layer 62, the thickness ratio of the color filter layer 62 is limited and improvement of the color purity is limited.
In FIG. 4B, in order to have a desired thickness ratio of the color filter layer 62, the transparent buffer layer 64 has a second thickness higher than the first thickness of FIG. 4A, and a high step at the borderline of the transmissive portion “tt” and the reflective portion “rr”. Since the color filter layer 62 is made of a viscous resin and is formed according to a surface of an underlayer, the color filter layer 62 also has a step at a top surface. Therefore, the difference “Δd” between the designed thickness d3 and the fabricated thickness d4 occurs, and improvement of the color purity of the transmissive portion “tt” is limited.
Accordingly, it is very difficult to form the transparent buffer layer having a color filter thickness to be the desired thickness in the transmissive portion, whereby the color difference occurs between the transmissive portion and the reflective portion of the DCF structure. If the color filter in the transmissive portion does not have a desired thickness to obtain the desired color purity, color reproduction of the transmissive portion will not increase as much as that of the reflective portion.
Moreover, when the transparent buffer layer has the high step at the borderline of the transmissive portion and the reflective portion, the color filter thereon has an uneven surface so that planarization of the common electrode formed on the color filter is degraded. Specifically, the uneven surface of the color filter causes the common electrode to have a rough surface, thereby deteriorating image display quality of the LCD device.